Flexible Alternating Current Transmission Systems (FACTS) are used for the dynamic control of voltage, impedance and phase angle of high voltage alternating current (ac) electrical power transmission lines. Since the concept of FACTS was introduced, a family of power electronic equipment has emerged for controlling and optimizing the flow of electrical power in electrical power transmission lines. This equipment makes use of large rating (4500 V to 6000 V, 4000 A to 6000 A) gate turn-off thyristors (GTOs) in high power inverter configurations that can be controlled to behave as three-phase sinusoidal voltage sources. FACTS controllers are operated synchronously with the transmission line and can be connected either in parallel, producing controllable shunt reactive current for voltage regulation, or in series with the line, for controlling the flow of power directly. The family of FACTS controllers includes the Static Synchronous Compensator (STATCOM), the Synchronous Series Compensator (SSSC), the Unified Power Flow Controller (UPFC), the Interline Power Flow Controller (IPFC), and the Generalized Unified Power Flow Controller (GUPFC). The basic theory behind the UPFC is described in L. Gyugyi, et al., “The Unified Power Flow Controller: A New Approach to Power Transmission Control,” IEEE Transactions on Power Delivery, vol. 10, pp. 1085–0193, 1995. The basic theories behind the IPFC and the GUPFC can be found in, respectively, S. Zelingher et al., “Convertible Static Compensator Project—Hardware Overview,” Proc. IEEE Winter Power Meeting, vol. 4, pp. 2511–2517, 2000, and B. Fardanesh et al., “Multi-Converter FACTS Devices: The Generalized Unified Power Flow Controller (GUPFC),” Proc. IEEE Winter Power Meeting, vol. 4, pp. 2511–2517, 2000.
For Voltage-Sourced Converter (VSC) based FACTS controllers, a maximum power transfer typically occurs when the controller is operated at its rated capacity (e.g., maximum current, voltage, or MVA rating). For STATCOM and SSSC devices, power transfer at rated capacities can be achieved by relaxing certain operating parameters or setpoints to ensure reliable power transfer. For UPFC, IPFC and GUPFC devices, however, a reliable power transfer at rated capacities is more complex because of the ability of these controllers to circulate active power.
One problem with operating a FACTS device at its rated capacity is the heightened risk of voltage collapse. Voltage collapse occurs when a power controller attempts to serve more load than the output voltage of the controller can support. Thus, a FACTS device operating at its rated capacity should provide maximum power transfer, while avoiding voltage collapse, i.e., maintain voltage stability.
Accordingly, what is needed is a power dispatch strategy for enabling a FACTS device to provide maximum power transfer and voltage stability, while operating at or below its rated capacity. Such a dispatch strategy should be easily integrated into existing FACTS designs, including without limitation UPFC, IPFC and GUPFC designs.